Page 15
ANTENNA CONSIDERATIONS
The choice of antennas is one of the most critical and often overlooked design
considerations. The range, performance, and legality of an RF link is critically
dependent upon the type of antenna employed. Proper design and matching of
an antenna is a complex task requiring sophisticated test equipment and a
strong background in principles of RF propagation. While adequate antenna
performance can often be obtained by trial and error methods, you may also want
to consider utilizing a professionally designed antenna such as those offered by
Linx. Our low-cost antenna line is designed to ensure maximum performance
and compliance with Part 15 attachment requirements. The purpose of the
following sections is to give you a basic idea of some of the considerations
involved in the design and selection of antennas. For a more comprehensive
discussion please review Linx applications note #00500 “Antennas: Design,
Application, Performance”.
THE TRANSMITTER ANTENNA
The transmitter antenna allows RF energy to be efficiently radiated from the
output stage into free space. In modular designs such as the HP, a transmitter’s
output power is often slightly higher than the legal limit. This allows a designer to
utilize an inefficient antenna in order to achieve full legal power while meeting
size, cost, or cosmetic objectives. For this reason a transmitter's antenna can
generally be less efficient than the antenna used on the receiver.
THE RECEIVER ANTENNA
A receiving antenna should give its optimum performance at the frequency or in
the band for which a receiver was designed, and capture as little as possible of
other off-frequency signals. The efficiency of the receiver’s antenna is critical to
maximizing range-performance. Unlike the transmitter antenna, where legal
operation may mandate a reduction in antenna efficiency or attenuation, the
receiver’s antenna should be optimized as much as is practical.
It is usually best to utilize a basic quarter-wave whip for your initial concept
evaluation. Once the prototype product is operating satisfactorily, a production
antenna should be selected to meet the cost, size and cosmetic requirements of
the product.
Maximum antenna efficiency is always
obtained when the antenna is at
resonance. If the antenna is too short,
capacitive reactance is present; if it is
too long, inductive reactance will be
present. The indicator of resonance is
the minimum point in the VSWR curve.
You will see from the following example
that antenna (A) is resonant at too low
a frequency, indicating excessive
length, while antenna (C) is resonant at
too high a frequency, indicating the
antenna is too short. Antenna (B),
however, is “just right.”
A
B
C
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